1. Field of the Invention
The embodiments of the invention generally relate to a method of optimizing photomask inspection that skips initial mask inspection based on etch mode, tool type, defect criteria, and etch area.
2. Description of Related Art
Semiconductor photomask manufacturing requires that all masks be defect and foreign material (FM) free for use in the semiconductor fabricator. Because a photomask pattern is repeated many times across a wafer, a defect on a mask can affect the image printed, resulting in wafer yield loss. In extreme cases, defects of this nature can produce a zero yield wafer. In photomask manufacturing, a mask is typically inspected for defects and foreign material two or more times during the process. If a defect occurs on a mask and the mask is inspected early in the process, defects can be repaired, or a new mask can be started early enough to meet customer serviceability requirements. While mask inspection is one of the most important steps in photo mask manufacturing, it is also one of the most time consuming and expensive processing steps. In order to manage cost and remain competitive in the marketplace there is a need to reduce the number of times a mask is inspected while still meeting serviceability requirements.
Mask inspection tools represent some the most complex systems used in the semiconductor industry, on par in cost and complexity with lithography expose tools and mask pattern generators. The complexity of mask inspection tools is driven by the need for high resolution and high speed in detection of defects. The first or initial inspection generally occurs after front end processing and prior to pellicle mount. This inspection checks for defects that occur during the print, develop and etch process. A final inspection occurs after a pellicle is mounted to assure no defects or foreign material were added during the pellicle mounting process. The initial inspection is done at a point where most of the processing of the mask has been completed and most defects would have been introduced. It is also done at a point where, if defects are found, they can be repaired, or another mask can be started and completed in order to meet manufacturing commitments or serviceability to the semiconductor fabricator. If, however, one can predict which masks are likely to be defect free after front end processing, one may skip the first inspection, saving significant time and money.
Inspection strategies exist today that allow initial inspection to be skipped based on defect size specifications and inspection tool availability. Defect size specification or defect criteria is the maximum size defect allowable on a mask in that it has no affect on the image printed on the wafer. Based on the current strategy, masks with large defect criteria may skip initial inspection if there is no inspection tool available when front end processing is complete. Because this current ‘skip’ inspection strategy is based only on defect criteria and available inspection tool capacity, it can miss many defects, resulting in pellicle demount for repair in many cases.